1. Field of the Invention
The present invention relates generally to an optical writer, an image forming apparatus, and a method of controlling the optical writer, and more particularly to phase correction of a pixel clock signal based on phase shift data.
2. Description of the Related Art
There is a recent trend toward promotion of digitized information, so that image processing apparatuses such as printers and facsimile machines used to output digitized information and scanners used to digitize documents are indispensable. These image processing apparatuses are often configured as multifunction peripherals usable as a printer, a facsimile machine, a scanner, and a copier by incorporating an image capturing function, an image forming function, and a communicating function.
In such image processing apparatuses, for those used to output digitized documents, electrophotographic image forming apparatuses are widely used. In the electrophotographic image forming apparatus, an electrostatic latent image is formed by exposing a photosensitive body to light, a toner image is formed by developing the electrostatic latent image using a developer such as toner, the toner image is transferred onto paper, and the paper is output.
In the electrophotographic image forming apparatus, an optical writer configured to expose the photosensitive body to light includes a light source configured to emit a beam to which the photosensitive body is exposed and a deflector for deflecting the emitted beam to scan the surface of the photosensitive body, such as a polygon scanner. In such an optical writer, a variation in the distance between the reflection surface and rotating shaft (axis of rotation) of the deflector causes a variation in the scanning speed of a light spot, which is the position of the beam emitted onto the scanned surface.
A description is given in detail of this variation in scanning speed. In common optical writers, an electrostatic latent image is written by exposing a photosensitive body to a light beam in accordance with a pixel clock signal. That is, the presence and the absence of light beam emission based on the presence or absence of color on a pixel basis are switched (determined) in accordance with a pixel clock signal. In principle, the frequency of this pixel clock signal is constant. Accordingly, the scanning speed of the light spot also is required to be constant on main scanning lines.
However, as long as the polygon scanner is used, it is natural for a variation to be caused in the distance between the reflection surface and rotating shaft (axis of rotation) of the deflector. Therefore, the above-described variation in speed is inevitable. That is, on a single main scanning line, the light spot moves fast in some parts and slow in other parts. Unless the variation in scanning speed is properly corrected, dot positions are caused to shift in the main scanning direction to cause fluctuation of an image, thus degrading image quality.
In order to correct this variation in scanning speed, the phase of the pixel clock signal on a scanning line is caused to shift in accordance with a variation caused in the scanning speed. Here, since a variation in the scanning speed is caused in each scanning line, the above-described phase shifting needs to be performed in each scanning line. However, if the scanning lines are subjected to phase shifting with the same timing, phase-shifted pixel clock pulses are successive in the sub scanning direction to produce an image with vertical streaks, thus degrading image quality.
In order to prevent the degradation of image quality due to this vertical-streak image, techniques are proposed to cause the start of correction to differ from scanning line to scanning line. (See, for example, Patent Documents 1 and 2 listed below.) According to the techniques disclosed in Patent Documents 1 and 2, the timing of the start of a correction due to phase shifting is randomized or is determined based on random numbers, thereby causing the timing of the start of the correction to differ from scanning line to scanning line to prevent generation of the vertical streaks.    [Patent Document 1] Japanese Laid-Open Patent Application No. 2007-203739    [Patent Document 2] Japanese Laid-Open Patent Application No. 2003-211723